xcb: Add support for obtaining default xcb connection

This is needed by qtx11extras now that we can meet a nullptr QScreen.

Change-Id: I755523cbbbbe188f454bd518d2d86a981542458f
Reviewed-by: Richard J. Moore <rich@kde.org>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -233,6 +233,9 @@ void *QXcbNativeInterface::nativeResourceForIntegration(const QByteArray &resour
     case Display:
         result = display();
         break;
+    case Connection:
+        result = connection();
+        break;
     default:
         break;
     }
@@ -454,6 +457,12 @@ void *QXcbNativeInterface::display()
     return Q_NULLPTR;
 }
 
+void *QXcbNativeInterface::connection()
+{
+    QXcbIntegration *integration = QXcbIntegration::instance();
+    return integration->defaultConnection()->xcb_connection();
+}
+
 void QXcbNativeInterface::setAppTime(QScreen* screen, xcb_timestamp_t time)
 {
     if (screen) {

src/plugins/platforms/xcb/qxcbnativeinterface.h

@@ -98,6 +98,7 @@ public:
     void *x11Screen();
     void *rootWindow();
     void *display();
+    void *connection();
     static void setStartupId(const char *);
     static void setAppTime(QScreen *screen, xcb_timestamp_t time);
     static void setAppUserTime(QScreen *screen, xcb_timestamp_t time);